In the manufacture of bevel and hypoid gears, the cutting tools utilized are primarily face mill or face hob cutters. The cutters generally comprise cutting blades formed from a length of bar stock material (e.g. high-speed steel or carbide) having a base or shank portion and a cutting end portion, including at least one cutting edge, at one end, or at both ends, of the base or shank. A plurality of cutting blades are usually arranged about a cutter head with the cutting ends of the blades projecting from a face of the cutter head. Such types of cutting tools are well known in the art of gear manufacture.
In face mill cutters, a plurality of cutting blades are arranged about a circle in a cutter head such that one tooth slot is formed with each plunge of the cutter and the cutter must be withdrawn and the workpiece indexed to the next tooth slot position in order to form the next tooth slot (i.e. intermittent indexing). Face milling cutters usually comprise alternating inside and outside cutting blades that cut, respectively, inside and outside portions of a tooth slot. Face mill cutters may also comprise successive cutting blades that remove stock material from the entire tooth slot, such as is disclosed in U.S. Pat. No. 1,236,834 to Gleason, U.S. Pat. No. 1,667,299 to Wildhaber or US 2007/0011855 to Ribbeck. If desired, one or more “bottom” cutting blades may be included for removal of stock material from the bottom or root portion of a tooth slot.
Face hobbing comprises cutting blades arranged about a cutter, not in line with each other, but in groups, with usually two or three cutting blades per group. In two-blade groups, such as disclosed by U.S. Pat. Nos. 4,575,285 to Blakesley; 4,621,954 to Kitchen et al. and 4,525,108 to Krenzer, the blade pair comprises an inner cutting blade and an outer cutting blade. In the three-blade group, such as disclosed by U.S. Pat. No. 3,760,476 to Kotthaus, a “bottom” cutting blade is included along with an inside and outside cutting blade. Unlike most face milling processes, in which all cutting blades pass through the tooth slot during its formation, face hobbing comprises each successive group of cutting blades passing through respective successive tooth slot with each blade in the group forming a cut completely along the longitudinal portion of the tooth slot. The cutter and the workpiece rotate in a timed relationship with each other thereby allowing continual indexing of the workpiece and continual formation of each tooth slot of the gear. Thus, in face hobbing, a single plunge of the cutting tool results in all tooth slots of the workpiece being formed (i.e. continuous indexing).
Cutting blades may be made of any suitable tool material such as conventional or powered metal hardened high speed steel (HSS) of any alloy composition (such as, for example, M2, M4, Rex 45, Rex 54, Rex 76, T15, Rex 121 or others) or made of carbide hard metal of any alloy composition, such as P and K grades. The wear surfaces of cutting blades may be coated (and recoated after sharpening) with PVD single or multi-layer coatings consisting of any commercially available wear coating or combination of wear coatings such as, for example, TiN, TiCN, TiAlN, AlTiN, CrAlN, ZrN, CrN and others. Gear cutting operations may be performed utilizing a coolant or lubricant (i.e. wet cutting) or may be carried out in the absence of any such coolant or lubricant (i.e. dry cutting). Dry cutting operations are usually performed utilizing cutting blades comprising carbide materials.
In face hobbing processes, and with most face milling processes, a left hand cutter (having blades which cut in a counterclockwise direction when viewing the back of the cutter) is utilized to cut a left hand gear. The “hand” (left or right) of a gear is the direction of inclination of the gear teeth as viewed from the face of the gear (ring gear or pinion for a bevel gear set) at the 12 o'clock position. Thus, it can be seen that since a left hand member of a gear set mates with a right hand member of the gear set, a right hand cutter is utilized to cut the right hand member of the gear set. Therefore, for cutting both members of a gear set, two cutters (left hand and right hand) are usually employed.
As mentioned above, the cutting blades of a face hobbing cutter are arranged in blade groups, with the blades of one group passing through one slot, while the blades of the following group pass through the following slot of the part. This cutting sequence requires a certain arrangement (regarding radial location and tangential offset) of the blades in one group, which repeats for each group. It can be understood that with respect to a left hand cutter, the cutter to cut a right hand part is a mirror image of the left hand cutter regarding cutting direction and blade orientation within one blade group. For example, a left hand pinion mates with a right hand gear, which in face hobbing requires a pair of cutters (one left hand and one right hand) in order to cut the two mating members (pinion and ring gear) of a bevel gear set.
The present invention eliminates the need for two cutter heads particularly for face hobbing. A single cutter head is proposed thereby reducing the expense of producing mating members of a gear set.